1. Field of the Invention
The subject matter disclosed, generally relates to automatic exposure control in a digital camera.
2. Background Information
Digital cameras contain an image sensor that converts optical energy into electrical signals. Most digital cameras are equipped with automatic exposure control. Exposure control involves determining the exposure settings that best utilize the dynamic range of the image sensor given the characteristics of an image scene being photographed. The dynamic range of an image sensor may be defined as the ratio of the highest signal output to the lowest useful signal output.
Image sensors have a limited dynamic range. A typical electronic image sensor has a dynamic range less than the dynamic range of natural scenes. This is mainly due to multiple light sources with widely varying intensities illuminating the scene objects.
Generally speaking it is desirable to have an exposure so that the captured image is not too bright or too dark. Commonly, this is achieved through moving an image histogram to an optimal point within the bounds of maximum and minimum output signal levels of the system. The histograms are frequency distributions of image pixels based on intensity values.
Exposure control in a camera typically involves measuring the average intensity of light from a sample area of an image scene being photographed. Exposure settings are then selected to scale the average to a pre-determined percentage of the dynamic range of the image sensor. One technique, commonly referred to as mean photometry, includes detecting the entire image region and controlling the exposure so that the detection signal has a constant level. Another method, commonly referred to as center-emphasizing photometry, detects only a central portion of the image region and controls the exposure so the detection signal has a constant level. A combination of mean photometry and center-emphasizing photometry can be accomplished by weighting the detection data of the entire video region and the detection data of the center region, and performing exposure control based upon detection data obtained by adding the weighted data at a fixed ratio. A finer exposure control can be achieved by sub-dividing a scene into areas, detecting video in each area and limiting the areas of detection data used in exposure control or changing the weighting. However, even the photometric methods described above do not always provide a state of exposure control that is suitable for the scene in question.